In recent years, there have been growing demands for the miniaturization of, for example, automobile engines, which has given rise to a need for small-sized gears. Simple size reduction makes gears incapable of withstanding loads such as rotating torque, and therefore many attempts have been made to improve the strength of gears. As one of the measures for improving the strength of gears, shot peening is widely known. Shot peening is a treatment for hardening the surfaces of objects such as gears by blasting their peripheral surfaces with shots such as steel balls by use of an impeller or nozzle.
FIG. 7 illustrates one example of shot peening utilizing a nozzle. In FIG. 7, shots 101 such as steel balls collide with the external surface of a gear 100. The gear 100 is so configured as to comprise (i) top lands 102, (ii) bottom lands 103, (iii) tooth flanks 104 each of which is formed, for example, by an involute curve extending from each bottom land 103 to each top land 102 and (iv) chamfers of tooth top 105 each positioned between each tooth flank 104 and each top land 102. The shots 101 are shot from a nozzle 106. The shooting direction 107 of the shots 101 from the nozzle 106 is substantially perpendicular to one of the bottom lands 103 of the gear 100 being rotated.
Such a conventional shot peening method, however, suffers from the problem that plastic deformation occurs in the tops of the teeth of the gear 100 and particularly on the ridge lines each of which is the intersection of each tooth flank 104 and each chamfer of tooth top 105, on account of the shock caused at the time of collisions of the shots 101, and this deformation leads to formation of small projections 108 on the tooth flanks 104. The projections 108 harm another gear which is in mesh with the gear 100, with the result that the service life of the gear is shortened. In addition, when the gears come into mesh with each other, they make a noisy action owing to the presence of the projections 108.
The present invention has been made for the purpose of overcoming the foregoing problems and therefore one of the objects of the invention is to provide a shot peening method for gears capable of hardening the surface of a gear without formation of projections on its tooth tops. Another object of the invention is to provide gears of high strength hardened by this shot peening method.